The present invention relates to an electric valve drive device which opens and closes an intake or exhaust poppet valve electrically.
A valve drive system in an ordinary engine mainly comprises a cam shaft, a rocker arm (or a tappet), a valve spring and a valve spring retainer. Rotational force of a cam of the cam shaft driven by a crank shaft is transferred to a poppet valve via the rocker arm to make opening/closing movement.
Output performance and fuel efficiency of an engine generally depend on intake and exhaust efficiency, and the higher they are, the higher engine performance is obtained owing to smooth gas exchange in a cylinder.
However, an automobile engine has broad rotation range so that it is difficult to increase engine performance over the whole operation range. If high speed performance is increased, low speed performance is decreased, and if low speed performance is increased, high speed performance is decreased.
To solve the problem, there is high-speed valve timing wherein lifts of intake and exhaust valves become larger with a larger overlapping range to increase intake/exhaust efficiency. There is also a small overlapping range valve timing wherein lifts become smaller to form a strong swirl at low speed where combustion is likely to be unstable.
Recently, to meet such requirements, an engine which has a valve operating mechanism of both low and high speed performance, or a variable valve timing lift mechanism which has two valve timing and lifts for low and high speeds is utilized.
However, the variable valve timing lift mechanism is basically similar to the mechanical valve operating mechanism which has the above cam shaft as the drive source. It is thus impossible to remove the inherent performance decrease factor of the valve operating mechanism or performance decrease factor caused by mechanical loss and followability of a valve to a cam.
Valve timing and lift are determined by phase and profile of a cam. It is impossible to vary them over the whole working range of the engine. There is no choice but to set two valve timings and lifts for low and high speed as above.
To solve the problem, as disclosed in Japanese Patent Laid-Open Pub. Nos. 10-37726 and 10-141928, it has been suggested to provide an electromagnetic valve drive device in which a valve is opened and closed by magnetic force instead of mechanical valve drive system having a cam shaft. But, in such an electromagnetic valve drive device, the valve is merely opened and closed by the attractive force of an electromagnet, thereby increasing seating noise and providing low responsiveness during valve operation.
Furthermore, because the control range for valve timing and lift is small, it is difficult to obtain optimum valve timing and lift corresponding to any working condition of an engine, and it would be impossible to improve engine performance over the whole working range.
There is a moving iron core or piece in the valve, so that inertial mass is increased during opening and closing of the valve to decrease responsiveness and reliability at control.
To overcome the disadvantages in the foregoing electromagnetic valve drive device, the applicant suggested an electric valve drive device in which a valve is driven by an electromagnetic actuator called a voice coil motor. A moving coil in the electric valve drive device is repeatedly moved at high speed in an axial direction to drive a valve directly, so it is necessary to increase mounting strength to improve durability and reliability. Also, it is necessary to connect the ends of the moving coil to an input terminal and to keep durability of a lead for connecting a fixed terminal to a terminal of the moving coil which always moves. In the electric valve drive device, electric current intensity in the moving coil is controlled to obtain optimum valve timing and lift corresponding to the working condition of the engine, and thus, it is necessary to cool the moving coil to increase durability.